The present invention relates to the medical field commonly referred to as Osteosynthesis, i.e., the fusion between segments of the spine and more particularly to a pedicle screw and rod system for immobilizing the segments during the fusion process.
Osteosynthesis is achieved by immobilizing the bone. When trying to achieve osteosynthesis and specifically fusion between different segments of the spine, one has to provide some type of immobilization. There are various prior art systems which try to achieve this purpose. The different systems involve placement of screws into the bone. The screws are then connected to each other by use of various sizes of rods or a plate. The bone segments that are being connected, especially in the spine, may be carrying different angles and different medical-lateral positions. Placement of a rod with a rigid screw or placement of a plate between two rigid screws is difficult because of the medial lateral displacement or angulation at different segments. One has to bend the rod or plate and at times achieve a complex bend in order to connect two different segments of the bone and especially two different areas in the spine. When dealing with the spine, the screws are ordinarily placed into the Pedicle, and due to the different positions of the pedicle and different angulations of the screw as it enters the pedicle, one encounters difficulty in positioning and connecting these screws at various points.
Even though one can create a complex bend at the rod or the plate in order to connect two or more screws, there are places in the pedicle where one runs the chance of stress risers at different points and breakage of the system as the bends can never be perfect.
A screw system, which is capable of accommodating the rod in a perfect location without creating any appreciable areas of stress riser, will alleviate some of the above problems. Such a screw system would allow the rod to be bent to achieve fixation between two different points while adjusting to any imperfections in the bend.
There is at least one polyaxial screw system that has been used in the past which will achieve some of these goals; however, there are some inherent problems with this particular system. This polyaxial screw has many components which makes placement of such a screw cumbersome, which in turn, lengthens the operative time for this particular procedure. The system has a locking screw on the inside as well as a locking nut on the outside of the housing, which causes the operation to take much longer to perform. The fixation point which will lock the polyaxial screw and keep it from angling once the system is tightened is also not ideal.
Several patents teach the use of a pedicle screw system which appear to provide several degrees of freedom (i.e., rotation and limited angular deflection about a fixed point) for the immobilization of bone segments. See for example U.S. Pat. No. 5,360,431 to Puno et al, U.S. Pat. No. 5,443,467 to Biedermann et al and U.S. Pat. No. 5,176,678 to Tsou. Each of these patented structures has certain drawbacks including the use of a conventional nut to secure the rod into place for support of the bone segments. The nuts have flat surrounding edges which are engaged by a wrench to tighten the nut. Due to the surrounding tissue, and the confined area, difficulty can arise in placing the nut in the correct position thus requiring even more time to perform the operation. During the operation the patient is under anesthesia and this extra time increases the risk to the patient. Also, when secured the nut protrudes into the surrounding soft tissue after the operation is completed. This protrusion can lead to irritation of the surrounding soft tissue and possibly inflammation.
Another problem arising with the use of the nut is the tightening process. The nut is secured through the use of a wrench. The wrench requires space around the nut to be operable which necessarily increases the scope of the surgical procedure. Furthermore, the wrench should not come into contact with the surrounding soft tissue to avoid the possibility of peripheral tissue damage. These limitations tend to further increase the risk to the patient during the operation.
Yet another problem with the prior art systems involves the manner in which the screw system is affixed to the rod. For example, systems like that disclosed in the Biedermann et al patent utilize washers or nuts on each side of the rod to secure the rod to the housing. The nuts or washers have planar surfaces which make contact with the rod only along segments of a line. Such a minimal contact with the rod will not provide sufficient purchase on the surface of the rod to prevent post operative movement between the rod and housing. Unless the screw housing is firmly grasps the rod the rod will have a tendency to twist or rotate inside of one or more of the housings. If the rod is allowed to rotate relative to one housing (and associated screw) while an adjacent housing and screw remain locked to the rod the screw secured to the locked housing may travel and break out of the vertebrae in which it was embedded. This will result in a damaged vertebrae and perhaps a severed nerve exiting the spinal column at that point. If the rod is allowed to rotate relative to all of the several housings to which it was originally secured it may migrate into muscle, soft tissue or even into the spinal column itself.
There is a need for a more reliable pedicle screw and rod system which may readily and rapidly be secured in place, with less bulky equipment and which is less intrusive to the surrounding soft tissue.
The present invention addresses the stabilization of bone segments through the use of a polyaxial pedicle screw assembly and rod. The rod is arranged to be secured between two or more embedded screw assemblies to immobilize segments of the spine. The assembly includes a screw which has a head and a threaded cylindrical shaft and is threaded into the bone. The head of the screw has a top and bottom, both of which are spherically convex in shape with the head being larger than the diameter of the cylindrical shaft. The top of the screw head has a wrench engaging surface, such as an allen wrench socket.
The screw fits within a polyaxial housing having a stepped bore adapted to receive the rod. The polyaxial housing is divided into three sections. The top section of the housing receives the entire screw including the head and is formed by a pair of spaced upstanding posts which define a U-shaped slot therebetween for receiving the rod. The inner walls of the posts are threaded for receiving a set screw which secures the rod in place. The middle section of the housing has an inner spherically concave surface for cradling the bottom of the head of the screw. The bore through the bottom section has a diameter which allows only the threaded cylindrical shaft to pass through. The screw, after insertion into the polyaxial housing, is threadably secured into the bone.
A washer with a generally spherically concave bottom surface to engage the head of the screw is then placed within the housing. Preferably the bottom surface of the washer is provided with a roughened surface, such as asperities in the form of sharp edges to provide a locking action between the washer and the screw head in the assembled condition. The top of the washer is provided with a concave surface, preferably semicylindrical or saddle shaped, to conform to the shape of the rod.
The lower surface of the washer provides for a positive gripping surface area with the screw head thereby adding to the stability of the rod and screw, once in place.
The screw assembly further includes a cup-shaped cap having two opposing openings to receive the posts and a cross-connector extending across the bottom of the cap. The cross-connector has a flat top and a bottom with a concave semicylindrical or longitudinal saddle shape to conform to the shape of the rod. The cup-shaped cap is adapted to be placed over the polyaxial housing with the bottom surface of the cross-connector making contact with the rod.
A set screw of conventional configuration is arranged to be threaded into the top section of the polyaxial housing by means of a wrench inserted into a wrench engaging surface, such as a allen wrench socket, in the top of the set screw, to tighten the assembly into place. The rod engaging surface of the cross connector may be provided with asperities formed, for example, by a series of sharp ridges (or edges) running parallel to the longitudinal axis of the rod or formed with a smaller radius of curvature at the bottom than at the top so that the cross connector will firmly grasp the rod and prevent it from twisting within the housing when the set screw is tightened.
With the set screw in place, but not tightened, the assembly has three degrees of freedom, i.e., rotatable and angularly positionable about the head of the screw. The tightening of the set screw secures the assembly into a single position. The set screw allows the assembly to be tightened while overcoming the disadvantage of potential soft tissue damage due to the use of a nut. The screw assembly of the present invention when secured in place does not protrude into the surrounding soft tissue and thus reduces the risk of irritation and soft tissue damage.
The present invention provides a highly flexible and stable bone segment immobilization system with a minimum number of components which results in a reduction in the time that a patient must remain under anesthesia.
The construction and operational features of the present invention may best be understood by reference to the following description taken in conjunction with the appended drawings in which like components in the several figures are identified by the same reference numerals.